Can I group items only in the Outliner
I had a need for something like this sometime back and as far as I know there is no way of doing this in Blender yet.

What I did was create and name some empties and then just parented the objects to it. To reduce clutter, you can keep the empties on a garbage layer and also reduce their size or disable relationship lines. Empties and their relationships to the child objects won't affect anything and you get to toggle the different 'groups' as needed.